This invention relates generally to microelectromechanical systems (MEMS). More particularly, it relates to sensing a control state of MEMS devices.
Previous patents and publications have described fiber-optic switches that employ moveable micromirrors that move between two positions. An example is shown in FIG. 1. Some of the prior art also employs electrostatic clamping of these mirrors at one or more of its two positions. For example, optical crossbar switches consisting of a series of moveable mirrors that are magnetically actuated are known in the art. The mirrors can be electrostatically clamped either in the horizontal position to the substrate or in the vertical position to the sidewalls of a separate chip. In the vertical position, the mirrors deflect light from an input fiber into an output fiber.
Previous work has described optical switches that use mirrors that are actuated between several discrete positions for switching light. These optical switches may rely on electrostatic comb drives to rotate the mirror. An electrostatic comb drive uses electrostatic forces between interdigitated fixed and movable comb members to rotate a device such as a mirror. It is possible to determine the relative angular position of the movable comb drive member with respect to the fixed comb drive member by measuring the capacitance between them. Unfortunately, comb drives have a limited range of angular movement and the capacitance may change only slightly over a desired range of discrete positions of the mirror. If each discrete position represents a different control state of an optical switch, it is difficult to correlate the capacitance measurement from the comb drive to the control state of the switch. Furthermore, not all optical switches use comb drive actuators.
Some of the prior art approaches require electrostatic clamping of a mirror structure to various electrodes in its different positions. For example, Behin et al. describe an optical crossbar switch consisting of a series of moveable mirrors that are magnetically actuated and can be electrostatically clamped either in the horizontal position to the substrate or in the vertical position to the sidewalls of a separate chip. Fujita et al. describe similar micromirrors that are electrostatically clamped against a shallow stop when deflected vertically.
FIG. 1 depicts an optical crossbar switch 100 that contains mirrors 102 that rotate between horizontal and vertical positions in order to switch optical signals 104 between one or more input fibers 106 and one or more output fibers 108. The mirrors 102 are typically rotated by a combination of magnetic force and mechanical torsion in a hinge member (not shown) at the axis of rotation of the mirror. Optical switches like that shown in FIG. 1 are described in detail in U.S. Pat. No. 4,580,873, entitled xe2x80x9cOptical Matrix Switch,xe2x80x9d Issued Apr. 8, 1986 to Frank H. Levinson, which is incorporated herein by reference. Comb drives are generally not used to actuate this type of switch because it is difficult to create a comb drive that could directly move the mirror over the desired angular range without some additional mechanical linkage. One means for fault detection in optical switches involves monitoring of the optical signals received by the output fibers 108. A splitter incorporated into the switch or fiber taps at the output fibers 108 can be used to monitor the output signals. This prior art method may also require monitoring of the input signal, since the criteria for failure is often a discrepancy between the input and the output signals. Unfortunately, monitoring the input and output optical signals incurs additional optical losses to the switch 100 since it requires tapping optical energy from the signals for monitoring. Furthermore, monitoring the input and output signals does not specifically indicate the cause of the failure, as the mirror position is not directly monitored.
There is a need, therefore, for an improved MEMS device with improved fault detection to directly detect faults in the control state of the mirror positioning mechanism.
Accordingly, it is a primary object of the present invention to provide microelectromechanical system (MEMS) device having a fault detection system that directly measures mirror control state.
The objects and advantages are attained by an apparatus and method that allow for detection of whether a rotatable MEMS element is in a first or second position, , e.g., horizontal or vertical, and whether it is properly clamped in either of these two positions. This sensing capability is useful for fault detection. By sensing the mirror position, mirror failure can be immediately detected, and traffic through the switch can be appropriately re-routed. Embodiments of the invention provide apparatus and methods for detecting whether mirrors used in a certain type of optical switch are in the xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d position. Specifically, this invention applies to switches that employ mirrors that move between an xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d position, wherein they reflect light from an input fiber into an output fiber in the xe2x80x9conxe2x80x9d position, and allow the light to pass in the xe2x80x9coffxe2x80x9d position. Electrodes are positioned in this system such that the mirrors are close to, and therefor capacitively coupled to, a different electrode depending on whether they are in the xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d position. This invention is especially useful for switches that already employ electrodes for electrostatic clamping of mirrors in one or more positions, since those same electrodes can be used both to electrostatically clamp the mirrors and to sense their position. The method described in this invention comprises sensing of the capacitance between the mirrors and the one or more electrodes used to clamp the mirrors in its one or more positions in order to detect which of the positions the mirrors are clamped in. Furthermore, the magnitude of the capacitances can be monitored to detect improper clamping. The apparatus may be incorporated into a MEMS mirror optical switch controlled by a computer processor.